This application is a national stage of PCT/DE99/00155 filed Jan. 19, 1999, and based upon German application 198 02 038.4, filed Jan. 21, 1998, under the International Convention.
The invention relates to a method and an apparatus for operating a direct methanol fuel cell with gaseous fuel.
A fuel cell is an electrochemical converter in which, without recirculation of heat and mechanical energy, electrical energy can be recovered directly from a fuel and an oxidation agent. It is comprised of a cathode, an anode and an electrolyte lying therebetween. The oxidation agent (e.g. air) is fed to the cathode and the fuel (e.g. methanol) is fed to the anode. The cathode and anode in the fuel cell have continuous porosity as a rule so that the two operating agents (fuel and oxidation agent) can reach the electrolyte and the reaction products carried away. Carbon dioxide is a typical reaction product at the anode side. At the cathode side water is produced.
Fuel cells are known from the publication DE 195 31 852 C1 in which proton-conductive membranes are used as an electrolyte and which are operated at temperatures of 100xc2x0 C. At the anode of such a fuel cell, protons are formed in the presence of the fuel by means of a catalyst. The protons traverse the electrolyte and bind at the cathode side with the oxygen stemming from the oxidation agent to form water. Electrons are thereby liberated and electrical energy produced.
Heat is generated in a fuel cell. This heat is, as a rule, carried off by a coolant flowing through the fuel cell. For this purpose a fuel cell is provided with coolant lines. The temperature of the coolant fed into the coolant lines lies below the working temperature of the fuel cell.
With a so-called xe2x80x9cdirect methanol fuel cell,xe2x80x9d methanol together with water are converted at the anode to protons and carbon dioxide. The protons pass the electrolyte and are converted in the aforementioned form at the cathode to water. The carbon dioxide is carried away out of the anode compartment in gaseous reaction products. The direct methanol fuel cell is hereinafter referred to as DMFC.
It is basically possible to operate a DMFC with a liquid or vapor form mixture of methanol and water.
It is a disadvantage in the operation of a DMFC with liquid fuel that carbon dioxide forms as a second gaseous phase. The carbon dioxide formed immediately proximal to the electrolyte must be discharged through the pores of the anode at the region of the reaction. This restricts the uniform supply of the liquid fuel to the electrolyte. The power output of the fuel cell is as a consequence reduced.
Along the transport path of the fuel through the anode compartment the proportion of the gaseous phase continuously increases. There thus arises a very different distribution of the two phases (liquid and gaseous) in the anode compartment.
The dropping volume proportion of the liquid phase (formed by the fuel and the water) along the anode compartment gives rise to an increasing vaporization of the original liquid phase.
The anode compartment divides itself into two regions by the aforementioned dropping volume proportion and the thus associated increased vaporization. In a first region, the electrolyte is wetted with a fuel/water mixture. In the second or following region the electrolyte is not wetted with liquid.
Transport of liquid and of gaseous substances in a porous system run quite differently. If a liquid and a gaseous phase adjoin one another substantially only a gas transport through the pores which are not wetted with liquid occurs. The pores filled with liquid are generally not traversed by gas because of the high pressure loss in the gas. The different conditions in the anode compartment give rise to power losses.
At a suitably high pressure in the anode compartment, the aforedescribed distribution into two regions can be counteracted.
A higher pressure in the anode compartment has the disadvantage of increasing the permeation of fuel and water through the electrolyte. Power losses are the result. A higher pressure in the anode compartment in addition mechanically stresses the electrolyte membrane and gives rise, inter alia, to rupture.
The aforedescribed effects based upon a higher pressure at the anode side can be avoided by a suitably high pressure on the cathode side. An effort must be made to suitably increase the pressure on the cathode side. The efficiency of the fuel cell is correspondingly reduced.
An increased pressure, however, influences positively the transport and electrochemical processes in the porous cathode layer. Thus the efficiency is increased.
The pressure on the cathode side of a fuel cell of the type DMFC with a membrane serving as the electrolyte which is chosen based upon the aforedescribed counteracting effects is preferably 200 to 250 kPa. Typically the pressure at the anode side of a fuel cell is 30 kPa less than that on the cathode side.
If a vapor form methanol-water mixture is introduced into the anode compartment of a DMFC, there is only one gaseous phase available. Two phases are thus avoided in a simple manner along with the above-mentioned drawbacks.
It is the object of the invention to provide a method for efficient operation of a DMFC and to provide an apparatus for carrying out the method.
The method of operating a direct methanol fuel cell of the invention uses the steps of:
evaporating a coolant in the fuel cell,
vaporizing a methanol-water mixture in an evaporator utilizing heat from the fuel cell, and
feeding the vaporized water methanol-water mixture to the anode compartment of the fuel cell.
Under the term xe2x80x9cevaporatorxe2x80x9d (evaporator unit), a device is to be understood in which the liquid evaporates.
In the method according to the claims, coolant is evaporated in the fuel cell. The evaporated coolant is fed in a first alternative to a heat exchanger. The heat exchanger is a component of an evaporator unit. By condensation in the heat exchanger, the heat is released at the evaporator unit at a substantially constant temperature. The phase transformation between vapor or gas and liquid is thus used to transmit heat from the DMFC to the evaporator. At the same time an approximately constant temperature is maintained.
In the evaporator unit, a methanol-water mixture is evaporated. The heat exchanger thus transfers heat to the methanol-water mixture. The evaporation temperature lies, during this operation, below the condensation temperature of the coolant used.
Suitable coolants are basically all liquids which can boil in the foreseen temperature range. This can be at ambient pressure (standard pressure) although it can also be the case in superambient or subambient pressure ranges.
For example the following:
The evaporated methanol-water mixture is then fed into the anode compartment of the fuel cell.
In a second alternative, the methanol-water mixture and the liquid coolant which is evaporated in the fuel cell are identical. The external evaporator with the heat exchanger are omitted. The fuel cell functions simultaneously as the evaporator. The evaporated mixture is then fed to the anode compartment.
With the process according to the invention, as a consequence, in an especially simple manner, a predetermined temperature can be maintained in the evaporator unit. With the method it can be ensured that no liquid superheating arises in the evaporator. A superheating can operate to cause a coking of the methanol. Coked methanol can no longer be transformed to hydrogen. The electric current yield is correspondingly reduced.
With the evaporation according to the invention of the methanol-water mixture before admission into the anode compartment of the fuel cell it can be ensured in an energetically effective manner that a liquid phase will not arise disadvantageously in the anode compartment. The heat which results from the operation of the fuel cell is then utilized for the evaporation. The system does not usually require any additional heat to be supplied.
To be able to convert the methanol completely, the molar proportion of the water-methanol water mixture must be at least equal to the molar proportion of ethanol. Advantageously as a rule, the molar proportion of water in the methanol-water mixture predominates.
When the molar mixing ratio of water:methanol=1.5, a complete evaporation at a total pressure of 10 kPa on the anode side is possible only when the temperature is significantly above typical operating temperatures of the DMFC. The desired heat flow from the fuel cell to the methanol-water mixture with consequent evaporation requires however only that the operating temperature of the DMFC lie above the evaporating temperature.
Liquids are completely evaporated when a temperature associated with a certain pressure is exceeded or the pressure is reduced below a pressure associated with a certain temperature.
The complete evaporation of the methanol-water mixture can thus be effected at a given temperature by adjustment of a suitable partial pressure of the mixture. In a preferred embodiment of the method, therefore, a carrier gas is added to the methanol-water mixture before the evaporation. The carrier gas is so selected that it is inert. CO2 is inert and is thus suitable. With the aid of the carrier gas, the partial pressure of the methanol-water mixture is so adjusted that a complete evaporation of the methanol-water mixture is achieved at a predetermined temperature. The predetermined temperature lies (somewhat) below the condensation temperature of the coolant.
In a preferred embodiment of the invention, the CO2 arising in the anode compartment is introduced as the carrier gas. It is thus fed to the evaporator unit. In this embodiment, inert carrier gas need not be separately prepared. This embodiment is especially disadvantageous in mobile applications.
The CO2 arising in the anode compartment can be used as a carrier gas for vaporization independently from the other method steps.
In a further advantageous embodiment of the method the fuel cell is cooled with a boiling liquid. This expression should be understood to mean that the liquid, already upon entry into the fuel cell is at boiling temperature.
Instead of being heated up in a fuel cell, the boiling liquid is transformed at approximately the same temperature into vapor. As a result, no temperature gradient develops in the fuel cell. Temperature gradients reduce the power output and the life of a fuel cell.
The apparatus of the invention encompasses an evaporator unit in which the methanol-water mixture is vaporized before entering the anode compartment of the fuel cell. The device has means for transferring the surplus heat of the fuel cell to the methanol-water mixture.
The apparatus can encompass means whereby the coolant leaving the fuel cell is fed to the heat exchanger. The discharged coolant is then fed to the heat exchanger of the evaporator unit and is used for vaporization of the methanol-water mixture. It can thus be ensured in an energetically satisfactory way that no disadvantageous liquid phase will arise in the anode compartment.
According to an advantageous embodiment of the apparatus, the apparatus has means to supply CO2 as a carrier gas to the evaporator unit, the CO2 arising in the anode compartment of the fuel cell.
In a further advantageous embodiment of the invention the coolant lines in the fuel cell serve simultaneously as the evaporator for the methanol-water mixture. The coolant is then fed to the fuel cell upon discharge from the coolant lines of the anode compartment. In this manner an especially high degree of efficiency is obtained.
Advantageously, in the last-mentioned embodiment, CO2 produced in the anode compartment is fed by corresponding means to the coolant lines.
In a further advantageous embodiment, the claimed apparatus has pumps for generating the desired pressures or flow velocities of the coolant.